The present invention relates to a device and method for assembling a plastic-lens-forming casting mold which is used when forming a plastic lens by casting polymerization.
Casting polymerization is known as a method of forming a plastic lens. A plastic-lens-forming casting mold used for casting polymerization usually includes a pair of molds which form the optical surfaces (convex surface and concave surface) of a plastic lens and a cylindrical casting gasket (to be abbreviated as a gasket hereinafter) in which the molds are fitted at a predetermined gap from each other and which defines the outer diameter of the lens. When forming the lens, first, the pair of molds are incorporated in the gasket to build the casting mold. Subsequently, a liquid monomer fills the internal space (cavity) of the casting mold. The casting mold is loaded in a heating furnace, and the monomer is heated and polymerized at a predetermined temperature for a predetermined period of time to harden. The casting mold is extracted from the heating furnace and released, so that a monomer molded product (lens) in the casting mold is released from the mold and extracted (see, e.g., Japanese Patent Publication No. 6-98631, Japanese Utility Model Laid-Open No. 55-134224, Japanese Utility Model Publication No. 6-39951, U.S. Pat. No. 4,251,474, Japanese Patent Laid-Open No. 4-232706, and “Spectacles” Jul. 1, 1987, p.p. 79-81, Kabushiki Kaisha Medical Aoi Shuppan).
The plastic lens manufacturing method described in Japanese Patent Publication No. 6-98631 includes a cylindrical gasket having a ring-like projection formed on its inner surface in the circumferential direction, and upper and lower molds to be incorporated in the gasket. The upper mold forms the front surface (convex optical surface) of a lens to be formed. When the upper mold is pushed into the gasket from above, its peripheral portion abuts against the projection, so that the upper mold is positioned. The lower mold forms the rear surface (concave optical surface) of the lens, and is pushed into the gasket from below by a pushing die. The pushing die which pushes the lower mold into the gasket includes a disk-like base and a cylindrical step projecting from the base. When the lower mold is placed on the pushing die, the step is inserted in the gasket, and the base abuts against the lower end face of the gasket, to define the push-in amount of the lower mold into the gasket.
The plastic-lens-forming casting mold described in Japanese Utility Model Publication No. 6-39951 includes a gasket made of a synthetic resin into an elastic cylindrical body, and a pair of molds to be incorporated in the gasket. The gasket has a ring-like projection projecting from its inner surface. The projection positions the pair of molds.
The plastic-lens-forming casting mold described in each of Japanese Utility Model Laid-Open No. 55-134224, Japanese Utility Model Publication No. 6-39951, and U.S. Pat. No. 4,251,474 includes a gasket having a ring-like ridge projecting from its inner wall surface in the circumferential direction, and a pair of molds. The pair of molds are fitted in the gasket and positioned by the ridge.
The plastic-lens-forming casting mold described in Japanese Patent Laid-Open No. 4-232706 includes a gasket and a pair of molds to be incorporated in the gasket. The gasket has a ring-like holding band formed on its inner wall surface, and a plurality of holding portions projecting at appropriate gaps in a circumferential direction. When one mold is incorporated in the gasket, it is positioned by the ring-like holding band. When the other mold is incorporated in the gasket, it is positioned by the plurality of holding portions.
The plastic-lens-forming method described in “Spectacles”, Kabushiki Kaisha Medical Aoi Shuppan, includes a small-height cylindrical gasket and a pair of molds to be incorporated in the gasket. The gasket has a ring-like projection which is formed on its inner wall surface in the circumferential direction. When the pair of molds are incorporated in the gasket, they are positioned by the projection.
When incorporating the pair of molds in the gasket, if a mold is inclined with respect to the axis of the gasket or the gap between the pair of molds is excessively large or small, lens thickness becomes nonuniform (prism defect) or insufficient to lead to a defective product. For this reason, various types of devices have been conventionally proposed to incorporate the two molds not to be inclined (for example, Japanese Patent Laid-Open Nos. 55-123430 and 2001-512383).
The method of forming a lens-forming casting mold described in Japanese Patent Laid-Open No. 55-123430 includes the step of positioning and holding two molds by a reference plate such that their lens-forming optical surfaces establish a predetermined positional relationship (reference positions for forming lens optical surfaces) and separating the two molds from the reference plate, the step of removing the reference plate, the step of setting a gasket at a position where the reference plate has been removed, and the step of returning the two molds to the reference positions for forming the lens optical surfaces and fitting them in the gasket.
According to the lens-forming casting mold described in JP Pat. Laid-Open No. 2001-512383, a rear casting mold (mold) which forms the concave surface of a lens is pushed into a gasket by a linear actuator.